1. Field of the Invention
This invention pertains to components of a high bit-rate monomode optical communication system present in a digital transmission system. More specifically, it pertains to optical channel waveguides in planar lightguide circuits which can be used to switch or power divide temporal optical soliton signals between waveguides prior to their entrance to, or at the exit from, optical fibers.
2. Prior Art Statement
In a modern optical communication system there are two aspects which limit performance. The first of these is optical attenuation due to absorption or scattering by the transmission medium. Attenuation limits how far a signal can travel in an optical fiber transmission line before it requires amplification. The second aspect is signal pulse broadening due to dispersion, which limits the bit-rate, or bandwidth, because of a loss of detector discrimination between adjacent pulses in a communication bit stream.
The aim of the present invention is only digital signal systems in which the signal consists of the presence or absence of pulses within a pulse-bit stream. It is not concerned with analog systems in which the signal consists of a varying amplitude of an electromagnetic wave.
The current practice for long distance optical communication systems requires the use of "repeaters" which involve two optoelectronic conversions. Photons of an optical signal travelling in the optical fiber are converted, through use of a photodetector, to an electric signal, i.e. electrons. The signal is electrically amplified to correct for absorption and electrically reshaped to correct for dispersion and the resulting signal converted back to photons, e.g. through use of a diode laser, for transmission through the next optical fiber link.
Recently, however, erbium doped optical amplifiers have been implemented into some fiber optic transmission systems. This innovation has the marked advantage that amplification, to correct for attenuation losses in long distance systems, occurs without the need to convert to electrons. The second problem, signal dispersion, is not addressed by these optical amplifiers.
Clearly there is considerable technological, as well as commercial, advantage in eliminating the periodic repeaters still required in an optically amplified fiber optic communication system to reshape signals which have broadened through dispersion. Long distance or high bit-rate digital communication applications would benefit from an optical system in which no signal pulse broadening due to dispersion occurs.
Dispersion, which leads to pulse broadening, has two components. The first is material dispersion which is a bulk property of the waveguide material system and its composition. The second is termed waveguide dispersion. It is a function of the waveguide's geometry, its dimensions and the profile of the material composition within the waveguide. Taken together the two components are generally termed chromatic dispersion.
To transmit signals over long distances or for high bit-rate transmission, in general, it is necessary that a pulse does not change shape with time. This in turn requires that there be a way to compensate for the naturally occurring pulse broadening due to chromatic dispersion within the optical transmission system.
Hasegawa, U.S. Pat. No. 4,406,516, discloses that a solution to this dispersion problem lies in a fiberguide communication system that propagates temporal optical solitons as the digital signal. A temporal optical soliton occurs when the pulse broadening due to chromatic dispersion is balanced by the contraction due to a nonlinear dependence of the transmission medium's index of refraction on light intensity. In '516 the conditions necessary to achieve a fiberguide communication system which can propagate temporal optical soliton pulses are disclosed. Hasegawa and Kodama, U.S. Pat. No. 4,558,921, disclose a repeaterless optical fiber communication system in which soliton pulse attenuation is non-electronically amplified by appropriate amounts at appropriate intervals. All of this prior art concerns fiberguides (round optical waveguides or optical fibers). Indeed, the design of the fiber aspects of a communication system has reached a high level of sophistication (Hasegawa and Kodama, Solitons in Optical Communications, Clarendon Press (1995)).
Inputting, and often outputting too, of digital signals to and from optical fiber transmission lines generally requires that the signals be processed in some way. Examples of signal processing include signal switching from one waveguide to another, power splitting of the signal, adding a signal to an existing bit stream or extracting a desired signal from an existing bit stream. Optical circuits which serve these processing functions are best fabricated in planar configurations using standard fabrication procedures and techniques developed for the processing of modern electronic integrated circuits. These optical circuits, generally termed planar lightguide circuits, have as a fundamental element a channel waveguide whose function is to transmit (propagate) the optical signal throughout the circuit. It is a consequence of the fabrication procedure that a channel waveguide will have a rectangular (or square) cross section. The prior art has dealt with waveguides having circular cross sections but not rectangular ones. Soliton propagation, being strongly dependent on the geometry of the waveguide, cannot be predicted for channel waveguides by following the criteria set forth for optical fibers.
Furthermore, because the digital signals are confined within a waveguide having two small dimensions and one large dimension, inventions based on spatial solitons have no bearing on the problems of soliton transmission through such waveguides. Temporal solitons are the vehicle for transmitting digital signals without pulse broadening, because they do not change their shape while propagating with time. Spatial solitons, in contrast, employ nonlinearity in optical properties to stabilize a beam shape spatially in a medium with three large, or at least two large, dimensions.
The fabrication of channel waveguides which will propagate temporal optical solitons has been disclosed in a companion application by the inventors, Bagley et al. in U.S. Ser. No. 09/169859, which is included herein by reference.
Two of the important processing functions in an optical communication system are switching a signal from one waveguide to another and power dividing an input signal to two output waveguides. These processes can be done electrically, for example, by converting the input photons to electrons through use of a photo detector and then electrically switching, or power dividing, the electronic signal to the desired output or outputs. In this case the signal must be converted back to photons, e.g. through use of a diode laser, for transmission through the next optical fiber link. An all-optical (all-photonic) device has the advantage that the photons need not be converted to electrons and then back to photons, as a result of which an all-optical device will, in general, be faster and less complex than an electrical switch (or power divider) for optical signals.
Silberberg and Smith, U.S. Pat. No. 4,856,860, disclose an all-optical switch which is appropriate for spatial solitons but not temporal solitons.
Goorjian, U.S. Pat. No. 5,651,079, discloses an optical switch which uses a medium that supports combined temporal and spatial solitons (light bullets) and requires a counter-propagating light bullet to switch the input light bullet signal.
Evans, U.S. Pat. No. 5,600,479, discloses a fiberguide based soliton switch in which the switched signal has a shifted central frequency. Evans, U.S. Pat. No. 5,717,797 also discloses a fiberguide based nonlinear optical loop mirror in which the unswitched signal is reflected back into the input.
Doran, U.S. Pat. No. 4,881,788, discloses a fiberguide based device requiring one splitter and one coupler in which switching is effected by means of an irtensity dependent phase difference between two parts of a divided input signal.
The problem presented in achieving a commercially and technically successful digital optical communication system for long distance communication or high bit-rate transmission is: to design not only optical fibers with necessary dimensions and optical properties and signal power to propagate temporal solitons, i.e. sustain temporal soliton transmission, but also to provide planar lightguide circuits containing channel waveguides which can propagate, switch (from one waveguide to another) and power divide (between waveguides) temporal optical solitons and which are compatible with optical fiber transmission lines. The present invention provides a solution to these latter problems.